Battery life is an important issue in, portable electronic devices. Mobile telephones, for example, often have power saving modes, such as a discontinuous receive mode, or DRX mode, to extend battery life. In the DRX mode, mobile telephones can have stringent power consumption objectives. In one example, the total power consumption in the DRX mode is expected to be below 10 milli-watts (mW) and targeted at a 2.5 mW level. Thus practitioners are driven to look at all power consuming functions included with the phones. One such device is the display and associated display processing circuits and drivers. Displays have become larger and more recently many displays are color displays, both factors increasing power consumption associated with the display. At the power consumption levels noted above, in many instances, only a simple, low quality, static image can be displayed. Also, there are power-saving conditions of a mobile telephone in which keeping video information alive has a higher priority than the quality of the displayed image.
One parameter that affects the amount of power consumed in displaying images is bits per pixel, or BPP. This parameter is also known as pixel depth. As the pixel depth increases, more bytes of video information have to be processed and transmitted through busses from memory to the display controller and to the display panel. Data shows that approximately 2.2 mW of power can be spent to move 16 bit QVGA format (parallel LCD interface) data from a video Static Random Access Memory (SRAM) buffer to a Liquid Crystal Display (LCD) controller and then to a corresponding display panel. Reducing the pixel depth to eight bits almost halves the power consumption (from 2.2 mW to 1.1 mW). A reduction of the pixel depth from twenty-four bits to eight or four bits provides even more power reduction.
LCD controllers typically have programmable pixel depth values. That is, a typical LCD controller has a palette RAM that can increase the pixel depth to improve the image quality, but the opposite is not true. That is, a typical LCD controller cannot reduce the pixel depth. To reduce the pixel depth, all video images must be re-encoded with CPU processing, which may require more energy than that saved by the reduction in pixel depth. That is, to change pixel depth from a higher level, such as sixteen, twenty or twenty-four BPP, to a lower value, such as four or eight BPP, all video images and primitives, which are already mapped for higher pixel depth into video memory and stored in part of the system memory, must be re-encoded. The energy cost of doing so may exceed the energy savings realized by lowering the pixel depth.